This study involves the characterization of the interactions between different photo-activated porphyrins and the tumor cell which lead to a high degree of selective toxicity. The clinically-useful agent being examined is a derivative of hematoporphyrin involving different types of tumors. Our studies are concerned with the isolation and characterization of active components of HPD, the study of binding of such fractions with tumor cells, and the delineation of the specific cytotoxic injuries to tumor cells resulting from photo-activation of cells containing HPD. These studies involve characterization of cell damage by both biologic and biophysical procedures, e.g., study of transport phenomena, membrane protein cross-linking, cell-surface charge & hydrophobicity, and fluorescence signals form porphyrins in cell-free systems and within tumor cells.